A numerical approach to optimize the performance of HTL-free carbon electrode-based perovskite solar cells using organic ETLs

Carbon electrode-based perovskite solar cells (c-PSCs) without a hole transport layer (HTL) have obtained a significant interest owing to their cost-effective, stable, and simplified structure. However, their application is limited by low efficiency and the prevalence of high-temperature processed electron transport layer (ETL), e.g. TiO₂, which also has poor optoelectronic properties, including low conductivity and mobility. In this study, a series of organic materials, namely PCBM ((Park et al., 2023; Park et al., 2023) [6,6]-phenyl-C61-butyric acid methyl ester, C72H14O2), Alq₃ (Al(C₉H₆NO)₃), BCP (2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline, C₂₆H₂₀N₂), C₆₀, ICBA (indene-C60 bisadduct, C₇₈H₁₆) and PEIE (poly (ethylenimine) ethoxylated, (C₃₇H₂₄O₆N₂)_n) have been numerically analyzed in SCAPS-1D solar simulator to explore alternative potential ETL materials for HTL-free c-PSCs. The presented device has FTO/ETL/CH₃NH₃PbI₃/carbon structure, and its performance is optimized based on significant design parameters. The highest achieved PCEs for PCBM, Alq₃, BCP, C₆₀, ICBA, and PEIE-based devices are 22.85%, 19.08%, 20.99%, 25.51%, 23.91%, and 22.53%, respectively. These PCEs are obtained for optimum absorber thickness for each case, with an acceptor concentration of 1.0 × 10¹⁷ cm^-3 and defect density of 2.5 × 10¹³ cm^-3. The C₆₀-based cell has been found to outperform with device parameters as V_oc of 1.29 V, J_sc of 23.76 mA/cm², and FF of 82.67%. As the design lacks stability when only organic materials are employed, each of the presented devices have been analyzed by applying BiI₃, LiF, and ZnO as protective layers with the performances not compromised. We believe that our obtained results will be of great interest in developing stable and efficient HTL-free c-PSCs.